FIELD OF THE INVENTION
The invention relates to a method for operating a firing circuit which is configured to actuate at least one firing cap of a firing system, in particular of a vehicle occupant protection system of a motor vehicle. The circuit has at least one firing capacitor for storing the energy necessary to fire the firing cap, and contains a switch which controls the connection between the at least one firing capacitor and the firing cap. The firing cap has a certain internal resistance and it requires, in order to be fired, a current which is at least as high as a predefined minimum current value for at least a specific minimum time period (T).
Firing circuits of this type are used to fire one or more firing caps when necessary, i.e., when it is desired that a pyrotechnic charge be ignited. Such firing circuits can generally be used to trigger explosive processes, but in the present invention they are preferably used in vehicle occupant protection systems of motor vehicles, in particular for securing systems, for example seat belt pretensioning systems or airbag systems.
U.S. Pat. No. 3,949,357 discloses a firing circuit in which the firing cap is arranged in series with two switches which are closed when the vehicle is subject to a collision. In other words, the switches serve as crash sensors. The firing circuit is fed by a battery which makes available the necessary firing energy.
U.S. Pat. No. 5,554,890 (German published patent application DE 44 09 019 A1) discloses a firing circuit that is provided with its own firing capacitor array which is charged to a predefined charge voltage during the normal operation of the motor vehicle, by a voltage or current generator which is fed from the motor vehicle battery. The firing capacitor array is composed in the simplest case of a single firing capacitor which is connected in parallel with the voltage or current generator, but it can also contain two or more firing capacitors which are connected in parallel or in series.
U.S. Pat. No. 5,929,535 (German patent DE 196 24 357 C1) discloses a firing circuit for an airbag in which either current is drawn from an energy accumulator in an unregulated fashion or in a regulated fashion with a regulated and limited output current mode.
The firing cap - also referred to as a squib - which is to be fired when the vehicle is in a collision is provided at the manufacturing end with specific specifications which define the minimum current value necessary for reliable firing and the minimum necessary period for which current must flow.
When the firing cap or firing caps have fired, there remains a residual quantity of energy which has not been consumed in the firing capacitor or capacitors. That residual quantity of energy cannot be utilized, but it makes it necessary for the firing capacitor or capacitors to be overdimensioned and thus be of a corresponding size.